JP4343558B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus that forms an image on both sides of a sheet.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a copying machine as an image forming apparatus of this type uses a sheet re-feeding unit and a sheet by a flapper or the like to transfer a transferred sheet conveyed through an image forming unit and a fixing unit when performing double-sided copying on the same sheet. It is configured to be introduced again into the image forming unit via the conveyance unit.
[0003]
When performing double-sided copying, the paper is positioned in the main scanning direction and the sub-scanning direction by aligning one side edge of the paper and the front edge of the paper to a specified position by a pair of horizontal registration means and a registration roller, respectively. Done.
[0004]
When the image is reduced, the image data is thinned out and read out from the image memory in each of the main scanning direction and the sub-scanning direction, the speed of the polygon motor and the operating frequency of the laser clock (CLK) are changed, and one pixel There is known a method for controlling the scanning line width on the document corresponding to the above (for example, see Patent Document 1).
[0005]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-014920
[0006]
[Problems to be solved by the invention]
However, the above-described conventional image forming apparatus has the following problems, and improvements have been demanded. That is, after the toner image is transferred in the copying unit of the copying apparatus, the paper is subjected to a fixing action in the fixing unit. At this time, when the heat fixing method is adopted, moisture in the paper is rapidly evaporated in the fixing unit. As a result, the vertical and horizontal dimensions of the paper change.
[0007]
The amount of paper shrinkage varies depending on the type of material (material), the size of the paper, the direction of the fibers in the paper, and also depending on environmental conditions such as humidity and temperature when the paper is stored. The amount of water changes, and the amount of shrinkage also changes accordingly.
[0008]
Further, in the conventional copying apparatus, when performing double-sided copying on the same sheet, the one side end (horizontal end) of the sheet in the horizontal direction and the leading end of the sheet are set to a specified position without considering the above-described shrinkage amount of the sheet. Since the laser emission reference is positioned so that an image is formed, the position of the image and the paper shifts as the distance from the specified position on the double-sided (backside) paper increases as the paper size changes. There was a problem that occurred.
[0009]
FIG. 28 is a diagram showing a positional deviation between a conventional image and a sheet. When the image is formed on the surface of the paper, since the paper is not contracted, it is possible to form (draw) a desired frame as shown in FIG. This frame is drawn from one end of the paper with the distance b in the sub-scanning direction and the distance a in the main scanning direction as the image writing reference position. For example, it is assumed that the sheet is shrunk due to the fixing action of the fixing unit, and the outer dimension of the sheet becomes 9/10.
[0010]
If the shrinkage due to the fixing unit is not taken into account when forming the image of the back surface, the size of the frame is the same as that of the front surface. Since the image writing position is at a distance a and b from one end of the paper, as in the front surface, the positional relationship of the frame with respect to the paper is as shown in FIG. Shift. At this time, since the entire sheet shrinks on the front surface, the positional relationship between the frame and the sheet is the same as that shown in FIG. However, the size of the frame is reduced by contraction.
[0011]
When the front and back images are seen through, the front and back frames are misaligned as shown in FIG. 4D, which is very unsightly. Further, even in the spread state as in the bookbinding mode, for the same reason, it becomes difficult to see because the size of the image is different between the right page and the left page. Therefore, when the image formed product becomes a product, its value is greatly reduced.
[0012]
As a first method for solving such a problem, when copying on both sides (back side), the reference position of the transfer paper is set in order to cope with the amount of change in the vertical and horizontal dimensions generated in the fixing unit. It has been proposed to displace a predetermined amount from a reference position in the surface copying process.
[0013]
FIG. 29 is a diagram showing the result of copying the back face while being displaced by a preset amount. As shown in the figure, although there is an effect of making the image misalignment inconspicuous, the size of the front frame and the size of the back frame are not different from each other. In addition, when the paper material (material) and the paper size are changed, the actual shrinkage amount of the paper does not match the preset change amount of the reference position, and it is difficult to obtain a desired effect. Further, since the amount of shrinkage of the paper greatly depends on the environmental conditions (humidity and temperature) in which the paper is stored, it is difficult to set the amount of change in advance. Although the positional relationship between the first sheet and the frame is improved, the size of the frame on the front surface and the back surface is different.
[0014]
Furthermore, in addition to the method of displacing the reference position of the transfer paper by a preset amount, a method of presetting the shrinkage amount of the paper and reducing the image on the back side according to the set shrinkage amount is also considered. Since the amount of shrinkage is not constant depending on the paper material, size, fiber orientation, storage environment, etc., it was expected that it would be difficult to obtain the desired effect.
[0015]
As a second method for solving the above problem, a change in the sub-scanning dimension of the paper is detected by a sensor (flag type sensor) using a mechanical sensing lever, and the latent image is based on the result. It is considered that the image size is corrected for paper shrinkage in the sub-scanning direction by finely adjusting the speed (process speed).
[0016]
FIG. 30 is a diagram showing a state in which a change in the paper in the sub-scanning direction is detected by a sensor (mechanical flag sensor) using a mechanical sensing lever. In this flag type sensor, the lever 1201 is arranged so as to block the paper path. When the lever 1201 is pushed by the leading end 1030 of the paper passing through the paper path, the lever 1201 rotates rightward in the direction of the arrow a, and the lever The front end position of the paper is detected by shielding the light beam of the photocoupler 1202 arranged in the vicinity of 1201.
[0017]
When detecting the rear end position of the paper, the lever 1201 rotates counterclockwise by its own weight or a spring when the paper passes, and the photocoupler 202 can receive light again. When detecting the rear end position of the paper, the movement (turning) time of the lever 1201 is determined by the weight of the lever and the spring, so that the bounce is likely to occur, and the detection is performed for a certain amount of time so that there is no false detection due to the influence. Need to continue. As a result, the detection time for the trailing edge detection is longer than that for the leading edge detection.
[0018]
Specifically, the processing time for detecting the rear end position is empirically required to be about 20 ms. When converted to a distance, assuming that the sheet conveyance speed is 500 mm / s, 20 ms × 500 mm / s = 10 mm. Become. Therefore, although it is effective for an image forming apparatus that shrinks by 10 mm or more, it is obvious that the image size cannot be corrected with high accuracy. In recent years, the demand for deliverables required in the POD (Print On Demand) market has increased, and the demand for printing accuracy on the front and back surfaces has increased greatly. The accuracy is shown in FIGS. 28 (A) and 28 (B). The requirement for the amount of deviation between the front surface and the back surface is within 0.5 mm, and further within 0.3 mm.
[0019]
Therefore, when the positional deviation due to thermal contraction due to fixing is corrected to be within 0.3 mm, it is necessary to increase the detection accuracy. In the above-described mechanical flag sensor, since the flag moving time and the processing time for preventing erroneous detection due to chattering are prolonged, the detection capability is not accompanied and an accurate contraction rate cannot be obtained. That is, the detection accuracy cannot reach the target of 0.3 mm. In recent years, it is conceivable to use an optical sensor instead of the mechanical flag sensor. However, since the detection accuracy of the optical sensor is about several millimeters in terms of distance at present, the mechanical flag sensor Compared with, detection accuracy is improved, but sufficient accuracy is not yet obtained. The main cause is restrictions such as the spot diameter, but details thereof are omitted here.
[0020]
Even if the measurement of the amount of shrinkage in the sub-scanning direction is realized to some extent by the above method, it is difficult to detect the amount of shrinkage in the main scanning direction with the same method. On the basis of this, it is conceivable to reduce or shrink the image by uniquely predicting or determining a dimensional change in the main scanning direction and performing image processing such as thinning out pixels in the main scanning direction accordingly. Also in this case, since the shrinkage rate in the main scanning direction is predicted from the dimensions in the sub-scanning direction, it is difficult to accurately correct the shrinkage depending on environmental conditions such as paper type, humidity, and temperature.
[0021]
Accordingly, the present invention provides an image forming apparatus capable of accurately correcting an image size in accordance with a change rate of the sheet size in the main scanning direction and the sub-scanning direction when forming images on both sides of the sheet. Objective.
[0022]
It is another object of the present invention to provide an image forming apparatus that is very advantageous in terms of cost and mounting by detecting the rate of change in sheet size in both the main and sub-scanning directions with a single (same) line sensor.
[0023]
[Means for Solving the Problems]
In order to achieve the above object, an image forming apparatus of the present invention includes an image forming unit that forms an image on a sheet, and the sheet is conveyed to the image forming unit on the surface, and an image is formed on the surface by the image forming unit. An image forming apparatus that forms images on both sides of the sheet, and passes a plurality of read pixels so as to be aligned in the width direction of the sheet. A sheet reading unit arranged in a region, a main scanning direction change rate detecting unit for detecting a change rate of a dimension in the main scanning direction of the sheet conveyed on the back surface by reading the reading pixel, and reading the reading pixel. A sub-scanning direction change rate detecting means for detecting a change rate of a dimension of the sheet conveyed on the back surface in the sub-scanning direction, and a main scanning direction of the detected sheet Main scanning direction magnification correction means for correcting the magnification in the main scanning direction of the image formed on the back surface of the sheet by the image forming means based on the change rate of the dimension, and the detected change in the dimension in the sub scanning direction of the sheet And a sub-scanning direction magnification correcting unit that corrects a magnification in the sub-scanning direction of an image formed on the back surface of the sheet by the image forming unit based on the rate. The sheet reading unit includes a plurality of chips that store a number of read pixels divided in the main scanning direction among the plurality of reading pixels, and a selection unit that selects at least one of the plurality of chips. The main scanning direction change rate detection means and the sub-scanning direction change rate detection means each read out read pixels in the chip separately selected by the selection means. It is characterized by that.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an image forming apparatus according to the present invention will be described with reference to the drawings.
[0025]
[First Embodiment]
FIG. 1 is a diagram illustrating a configuration of an image forming apparatus according to the first embodiment. The image forming apparatus includes an image forming apparatus main body 10, a folding device 40, and a finisher 50. The image forming apparatus main body 10 includes an image reader 11 and a printer 13 that read a document image.
[0026]
A document feeder 12 is mounted on the image reader 11. The document feeder 12 feeds the documents set upward on the document tray 12a one by one from the top page in order to the left in the figure, and conveys them onto the platen glass through a curved path. The original is read by stopping at the position and scanning the scanner unit 21 from the left side to the right side in this state. After reading, the original is discharged toward the external paper discharge tray 12b.
[0027]
The original reading surface is irradiated with light from the lamp of the scanner unit 21, and reflected light from the original is guided to the lens 25 via the mirrors 22, 23 and 24. The light that has passed through the lens 25 forms an image on the imaging surface of the image sensor 26.
[0028]
Then, the entire original image is read by conveying the scanner unit 21 in the sub-scanning direction while reading the original image by the image sensor 26 line by line in the main scanning direction. The optically read image is converted into image data by the image sensor 26 and output. Image data output from the image sensor 26 is subjected to predetermined processing in an image signal control unit (image processing circuit) (not shown) and then input as a video signal to an exposure control unit (laser control circuit) (not shown) of the printer 13. To do.
[0029]
The exposure control unit of the printer 13 modulates laser light output from a laser element (not shown) based on the input image data, and the modulated laser light is scanned by the polygon mirror 355 while the lens 28 is scanned. , 29 and the mirror 30 are irradiated onto the photosensitive drum 31.
[0030]
An electrostatic latent image corresponding to the scanned laser beam is formed on the photosensitive drum 31. The electrostatic latent image on the photosensitive drum 31 is visualized as a developer image by the developer supplied from the developing device 33. Also, at the timing synchronized with the start of laser light irradiation, paper is fed from each cassette 34, 35, 36, 37, manual paper feed unit 38 or double-sided conveyance path, and conveyed to the image forming unit via registration rollers. Is done.
[0031]
This sheet is conveyed between the photosensitive drum 31 and the transfer roller 39, and the developer image formed on the photosensitive drum 31 is transferred onto the sheet fed by the transfer roller 39. The sheet on which the developer image is transferred is conveyed to the fixing unit 32, and the fixing unit 32 fixes the developer image on the sheet by heat-pressing the sheet. The sheet that has passed through the fixing unit 32 is discharged from the printer 13 to the outside (the folding device 40) through a flapper and a discharge roller.
[0032]
Here, when the sheet is discharged with its image forming surface facing downward (face-down state), the sheet that has passed through the fixing unit 32 is once guided into the reversing path by the switching operation of the flapper, and the trailing end of the sheet. After passing through the flapper, the paper is switched back and discharged from the printer 13 by the discharge roller.
[0033]
In addition, when a hard sheet such as an OHP sheet is fed from the manual sheet feeding unit 38 and an image is formed on this sheet, the image forming surface is faced up (face-up state) without guiding the sheet to the reverse path. ) Is discharged by the discharge roller.
[0034]
Furthermore, when double-sided recording for image formation is set on both sides of the paper, the paper is guided to the reverse path by the flapper switching operation, and then transported to the double-sided transport path. Then, the sheet is fed again between the photosensitive drum 31 and the transfer unit at the timing described above.
[0035]
The paper discharged from the printer 13 is sent to the folding device 40. The folding device 40 performs a process of folding the paper into a Z shape. For example, if the sheet is an A3 size or B4 size and the folding process is designated, the folding apparatus 40 performs the folding process. In other cases, the sheet discharged from the printer 13 passes through the folding apparatus 40. It is sent to the finisher 50. The finisher 50 is provided with an inserter 90 for feeding a special sheet such as a cover sheet or a slip sheet to be inserted into a sheet on which an image is formed. In the finisher 50, bookbinding processing, binding processing, punching, and the like are performed.
[0036]
Here, the photosensitive drum is used as the image carrier of the image forming apparatus, but a photosensitive belt may be used.
[0037]
[Heat shrinkage at fixing section]
The sheets stacked in the cassettes 34, 35, 36, and 37 contain a lot of moisture, and the amount of moisture depends on the environmental conditions such as temperature and humidity of the place where the image forming apparatus is disposed. Different. When these moisture-containing sheets are fed from each cassette, the developer image is transferred to the front (front) surface (first surface) by the photosensitive drum unit and the transfer roller unit. There is no shrinkage of the paper. Thereafter, when the fixing operation is carried out by heating and the fixing operation is performed by heating, the moisture contained in the paper evaporates all at once. As a result, the distance between the fibers in the paper is shortened, so that the whole paper contracts.
[0038]
Further, in order to form an image on the back surface (second surface), the paper is transported to a double-sided transport unit and further transported to a photosensitive drum unit and a transfer roller unit. Shrinkage due to heat gradually returns to the original with time, but does not return to the original paper length before forming an image on the back surface. When an image is formed on the back surface by the same control as the front (front) surface in such a paper state, a printed matter having different front and back magnifications is produced.
[0039]
Therefore, in the present embodiment, it is possible to correct the apparent size and positional relationship of the images formed on the front surface and the back surface by controlling the laser light according to the magnification of the reduced paper and reducing the image. Is possible. Details of the reduction ratio detection method and the laser light control method will be described later.
[0040]
[CIS layout, paper feed timing, and image export timing]
FIG. 2 is a diagram showing a print position adjusting mechanism arranged in a paper conveyance path leading to the photosensitive drum. In the figure, reference numeral 205 denotes a paper transport path. Reference numeral 206 denotes a circulation path. Reference numeral 31 denotes the photosensitive drum described above. A laser element 202 forms a latent image on the photosensitive drum 31. The arrangement of the laser element 202 is drawn for convenience and is different from the actual arrangement. Reference numeral 203 denotes a paper transport roller (registration roller). The paper transported along the paper transport path 205 stays in a state where it is once abutted, and is then fed to the photosensitive drum 31 side in accordance with a predetermined paper transport timing. .
[0041]
Reference numeral 204 denotes an image reading sensor (image sensor) that reads an image in order to detect the end of the paper, and is composed of a photoelectric conversion element array such as a CCD or CIS. In this embodiment, a CIS (contact image sensor) is used.
[0042]
In order to obtain the thermal contraction rate of the paper at the fixing unit, the paper size is measured twice in total, the time before the front side (first surface) image formation and the time when the paper passes the fixing unit after that. There is a need to measure. For this reason, the CIS 204 is disposed at the position shown in FIG. In this embodiment, the CIS is used not only for measuring the reduction ratio of the paper due to heat, but also for detecting the position of the paper with high accuracy when controlling the laser writing position. The position of the CIS shown in FIG. 2 satisfies such a requirement. Of course, if the CIS is dedicated to the reduction ratio detection and is not used for the purpose of detecting the position of the paper, it may be arranged at a position that does not satisfy the above requirement.
[0043]
The CIS 204 is disposed on the registration roller 203 side that is separated from the photosensitive drum 31 by the distance L1, and is disposed on the registration roller 203 side that is separated from the image forming point (point a) described later by a distance L2. Further, the CIS 204 is disposed away from a BD detector 108 described later by a distance L3 in the direction perpendicular to the paper feed direction.
[0044]
Reference numeral 108 denotes a beam detector (BD) detector that detects the irradiation timing of a laser element (simply referred to as a laser) 202. After the laser beam is irradiated to the BD detector 108 by the polygon mirror and then shaken and irradiated onto the photosensitive drum 31, a latent image is formed on the photosensitive drum 31.
[0045]
In the figure, point a indicates an image forming point. For example, when the image is formed by the laser 202 at the timing when the sheet passes point a by 5 mm, the rotation of the photosensitive drum 31 and the conveyance of the sheet 107 are performed in synchronization, and as a result, the output image is 5 mm from the leading edge of the sheet. Formed in position.
[0046]
In the figure, point b represents a transfer point, and point c represents a laser writing point. When a latent image is formed on the photosensitive drum 31 by the laser 102 at the laser writing point c, the toner is transferred onto the sheet at the transfer point b via the developing unit, and image formation is performed.
[0047]
When the image is formed, the sheet 107 sent out from the registration roller 203 is conveyed to the photosensitive drum 31 side along the sheet conveying path 205, and when the leading edge is detected by the CIS 204, the sheet 107 advances by a distance L2. Control is performed so as to irradiate the laser beam. Specifically, the time that the sheet 107 travels the distance L2 is counted by a timer, and when the time has elapsed, the photosensitive drum 31 is irradiated with laser light.
[0048]
In order to adjust the laser writing position with higher accuracy, timing in the paper feed direction (for convenience, the sub-scanning direction) and timing in the direction perpendicular to the paper feed direction (for convenience, the main scanning direction). Therefore, it is necessary to control writing by laser light according to the detection information.
[0049]
That is, after the leading edge position of the paper is detected by the CIS 204, the image formation start time is determined, and when the paper advances by the distance L2, the writing by the laser is started to adjust the image writing position in the sub-scanning direction. can do. Therefore, the distance L2 is a period from when the CIS 204 detects the leading edge of the sheet 107 to when a deviation in the direction perpendicular to the sheet feeding direction is detected and the timing of writing the laser beam in each direction is set. It is necessary to have at least a distance corresponding to time.
[0050]
In a normal image forming apparatus, the sheet conveyance speed and the rotation speed of the photosensitive drum 31 are set equal. This is because a distance L1-L2 from a position (image formation point a point) advanced from the CIS 204 by a distance L2 to a transfer position (transfer point b point) to the sheet that is a nip position between the transfer roller 39 and the photosensitive drum 31. This means that the circumferential (circular) distance on the photosensitive drum 31 from the laser writing position (writing point c point) to the sheet transfer position (transfer point b point) is equal.
[0051]
When the lateral edge position (lateral registration) of the paper is detected by the CIS 204, the distance x from the lower edge of the CIS 204 to the lateral edge position of the paper is added to the distance L3 from the beam detector (BD) 108 to the lower edge of the CIS 204. The distance (x + L3) is calculated, and after the laser beam is detected by the beam detector 108, the laser beam is shaken in the main scanning direction by the calculated distance, and then writing by the laser is started. The image export position can be adjusted.
[0052]
Such adjustment of the image writing position in the sub-scanning direction and main scanning direction by the laser light is performed by a timing control unit (TCU) 105 described later. That is, the TCU 105 turns on the registration roller 203 to start conveyance of the sheet, and then outputs a writing start timing to the laser control circuit 27 based on the detection signal from the CIS 204. The laser control circuit 27 drives the laser element 202 based on the image data sent from the image processing circuit (not shown) in synchronization with the writing start timing output from the TCU 105.
[0053]
With such a configuration, it is possible to control the image writing position after the shrinkage rate of the paper due to heat is determined.
[0054]
[Configuration of CIS]
FIG. 3 is a diagram illustrating the configuration of the CIS 204. The CIS 204 includes an image reading unit 205 and an LED light emitting unit 206. The image reading unit 205 includes a plurality of chips (1 to n) 211 to 217 in which a light receiving element unit and a shift register are accommodated in one chip, a selector 219, and an output unit 220. In this embodiment, the number of chips is seven (n = 7). Each light receiving element portion in each chip is provided with 1000 reading pixels.
[0055]
Of the 7,000 effective pixels in the entire CIS, 1000 reading pixels in 213, which is one of the seven chips, are used for reading in the sub-scanning direction (tip to be described later). In this embodiment, the reading in the sub-scanning direction is the chip 213. However, any of the chips 211 to 217 may be selected. On the other hand, 6000 reading pixels in the remaining 6 chips 211 to 217 are used for reading in the main scanning direction (lateral end detection described later). The number of effective pixels that is the sum of the plurality of chips is merely an example, and is not particularly limited, and may be an arbitrary number. Further, the chip division is not limited to 1: (n−1) in the present embodiment, and any number of divisions may be used. Moreover, you may use without dividing | segmenting.
[0056]
In the image reading unit 205, when the selector 219 effectively selects only a specific chip, for example, the chip 213 used at the tip, by the selector signal from the TCU 105, the image signal detected by the light receiving element unit 213a is loaded from the TCU 105. The signal (CIS-SH) is once read to the shift register 213b, and then sequentially transferred from the shift register 213b to the output unit 220 via the selector 219 in accordance with the clock (CLK) from the TCU 105. The output unit 220 converts the transferred serial image signal into parallel data and outputs it as CIS data.
[0057]
Further, when the selector 219 effectively selects the chips 211 to 217 to be used for detecting the lateral edge by the selector signal from the TCU 105, the image signals detected by the respective light receiving element portions 211a to 217a are temporarily stored by the load signal from the TCU 105. After being read out to the shift registers 211b to 217b, the data are sequentially transferred from the shift registers 211b to 217b to the output unit 220 via the selector 219 in accordance with the clock (CLK) from the TCU 105. The output unit 220 converts the transferred serial image signal into parallel data and outputs it as CIS data.
[0058]
On the other hand, the LED light emitting unit 206 includes an LED unit 221 in which a plurality of LED groups connected in series are connected in parallel, and an LED current adjusting circuit that is connected to the cathode side of each LED group and adjusts the current flowing through each LED group. 222. The LED current adjustment circuit 222 adjusts the entire LED light emission amount of the LED unit 221 according to the light amount control data from the TCU 105.
[0059]
FIG. 4 is a diagram showing the arrangement of the CIS 204 with respect to the paper passage area. The CIS 204 is arranged so that a plurality of read pixels are arranged in a direction perpendicular to the conveyance direction of the sheet 107. Further, the CIS 204 is disposed at a position where both ends of the paper to be detected in the main scanning direction can be detected. In the present embodiment, the case where the number of CIS read pixels is 7000 is shown. However, the number of read pixels is not limited to 7000, and the both ends of the maximum sheet to be conveyed are determined based on the resolution of CIS. It is sufficient that the number is sufficient for detection. That is, the plurality of reading pixels need to be arranged to be larger than the dimension in the main scanning direction of the conveyed sheet.
[0060]
In addition, since multiple pixels are used in the main scanning direction as pixel data used for leading edge detection, a sensor for leading edge detection is required compared to conventional single optical sensors and mechanical paper detection sensors. Therefore, the image forming apparatus can be made more compact by reducing the number of parts.
[0061]
And by detecting the lateral edge and then the trailing edge after the detection of the leading edge detection, it is possible to adopt different methods as the respective detection methods, and the detection accuracy suitable for each detection can be improved. It becomes possible. In particular, in front / rear end detection, using data of a plurality of pixels in the main scanning direction contributes to improvement in detection accuracy.
[0062]
Furthermore, by separately performing the front / rear end detection and the lateral end detection, it is possible to set the detection process to the shortest time with an optimal detection cycle.
[0063]
[Paper edge detection method using CIS]
FIG. 5 is a diagram showing a method of detecting the size of the paper in the main scanning direction and the sub-scanning direction using a single CIS. First, the leading edge of the sheet 107 being conveyed passes through a predetermined Chip_n in the CIS 204 to detect the leading edge of the sheet (see FIG. 1A). Thereafter, the sheet 107 is further conveyed, and both ends of the sheet 107 in the main scanning direction are detected by at least two or more chips (Chip) at a predetermined timing (see FIG. 5B). In FIG. 5B, two chips, Chip_n + x and Chip_ny, are used for detection. Thereby, the dimension of the sheet 107 in the main scanning direction can be obtained. Further, the sheet 107 is conveyed, and the passage of the rear end portion of the sheet 107 is detected by a predetermined Chip_n in the CIS 204 (see FIG. 3C). Thereby, the dimension of the sheet 107 in the sub-scanning direction can be obtained together with the result of the leading edge detection of the sheet previously obtained in FIG. As described above, by using a single CIS that is one line sensor, it is possible to obtain the sheet size in the main scanning direction and the sub-scanning direction. In the figure, “◯” indicates the detected part.
[0064]
[Configuration of control circuit]
FIG. 6 is a block diagram showing the configuration of the control circuit. The control circuit 51 includes an image processing circuit 52, a laser control circuit (V-CNT) 27, and a timing control unit (TCU) 105. The image processing circuit 52 is provided with an image memory (P-MEM) 56 that stores image data read by the image sensor 26, and a CPU 57 that processes the image data stored in the image memory 56.
[0065]
The laser control circuit 27 outputs a drive signal to the laser element 202 based on a signal output from the image processing circuit 52 according to the image data. The drive signal is output to the laser element 202 in synchronization with the timing signal from the TCU 105. The TCU 105 outputs a CIS control signal to the CIS 204, inputs CIS data read by the CIS 204, and outputs a timing signal to the laser control circuit 27 based on the CIS data. This timing signal includes a signal (registration ON signal) for driving the registration roller 203 in addition to the laser write signal of the vertical synchronization signal VSYNC, clock VCLK, and horizontal synchronization signal HSYNC.
[0066]
[Configuration of reduction rate detection circuit]
FIG. 7 is a block diagram showing the configuration of the TCU 105. The TCU 105 includes a counter 61, a registration ON unit 62, a front / rear end detection unit 63, a lateral end detection unit 64, a CIS controller 65, a front / rear end error detection unit 67, a lateral end error detection unit 69, and a sequence end. A setting unit (SEQ END) 70 and a correction parameter storage unit 71 are included.
[0067]
The counter 61 is activated by a sequence start signal (SEQ START) and counts clocks with a fixed period. The registration ON unit 62 turns on / off the driving of the registration roller 203. The sheet length in the sub-scanning direction is calculated based on the data on the leading edge position and the trailing edge position of the sheet, and the leading edge / rear edge detecting unit 63 is based on the CIS data input from the CIS 204. The rear end position is detected. Similarly, the horizontal edge detection unit 64 detects the horizontal edge position of the paper based on the CIS data input from the CIS 204. The paper length in the main scanning direction is calculated based on the horizontal edge positions at both ends of the paper.
[0068]
The CIS controller 65 outputs CIS control signals such as a load signal (CIS-SH), a clock (CIS-CLK), a selector signal, and light amount control data to the CIS 204.
[0069]
The leading edge error detection unit 67 generates an error signal (ERR) when the leading edge position of the paper detected by the leading edge detection unit 63 is out of a predetermined range. Similarly, the lateral end error detection unit 69 generates an error signal (ERR) when the lateral end position of the paper detected by the lateral end detection unit 64 is out of a predetermined range. The sequence end setting section 70 is set with a sequence count value for ending printing of one sheet. The correction parameter storage unit 71 may store correction values of the laser writing position in the main scanning and sub-scanning directions obtained by the adjustment process at the time of installing the CIS. Further, the correction unit parameter storage unit 71 stores a reduction rate (thermal contraction rate) Sh in the main scanning direction, which will be described later, and a reduction rate (thermal contraction rate) Sv in the sub-scanning direction.
[0070]
FIG. 8 is a block diagram showing the configuration of the tip detection unit 63. The tip detection unit 63 includes a plurality of edge circuits (EDDGE) 81, a timing generation circuit 82, and a counter 83. Register signals (REG1 to REGn) that specify pixel positions in the light receiving element portions 211 to 7a of the CIS 204 are input to each edge circuit (EDDGE) 81 together with the CIS data. When “no paper → paper present” is detected at the designated pixel position in synchronization with the count signal from the counter 83, the edge circuit (EDDGE) 81 generates edge (EDDGE1 to n) signals.
[0071]
The timing generation circuit (TIMING) 82 performs an averaging process on the plurality of generated edge (EDDGE1 to n) signals and outputs a tip detection signal (VREQ). Further, when performing tip detection, only a specific pixel may be used, but in this embodiment, the influence of noise or the like is removed by using a plurality of pixels. In addition, since tip detection uses a plurality of pixels, tip detection accuracy is further improved as compared with conventional single optical sensors and mechanical sensors.
[0072]
The counter 83 outputs a count signal to the plurality of edge circuits (EDDGE) 81 based on the load signal (CIS-SH) and the clock (CIS-CLK).
[0073]
[Paper feeding / image formation sequence]
FIG. 9 is a timing chart showing the operation of the TCU 105. The sheet 107 is conveyed to the registration roller 203 along the sheet conveyance path 205, and the sheet feeding / image forming sequence of this embodiment is started in a state where the sheet 107 stays on the registration roller 203. When the sequence start signal (SEQ START) is input to the counter 61, the counter 61 starts measuring a clock having a fixed period. When the count value of the counter 61 reaches timing a, the registration ON unit 62 sets the registration signal to H level and drives the registration roller 203 to ON.
When the count value reaches timing b, the operation in the tip detection mode in the CIS 204 is started. In the leading edge detection mode, as described above, a plurality of edges are detected, the averaging process is performed, and the leading edge of the sheet is accurately detected.
[0074]
When the leading edge of the sheet is detected when the count value reaches the timing c, the leading edge detection unit 63 outputs a leading edge detection signal VREQ to the CIS controller 65 and starts the operation in the lateral edge detection mode in the CIS 204. When the CIS controller 65 outputs a vertical synchronization signal VSYNC corresponding to the tip detection signal VREQ to the laser control circuit 27, the laser control circuit 27 considers the vertical margin based on the vertical synchronization signal VSYNC from the CIS controller 65 and performs laser processing. The writing position in the sub-scanning direction is adjusted. FIG. 10 is a diagram showing adjustment of the writing position by the laser. If the leading edge position of the sheet is not detected even when the count value reaches timing c ′ (c ′> c), the CIS controller 65 outputs a leading edge error signal (leading edge ERR).
[0075]
When the horizontal edge position of the sheet is detected when the count value reaches the timing d, the horizontal synchronization signal HSYNC and the clock VCLK are output to the laser control circuit 27. The laser control circuit 27 sets the writing position in the main scanning direction by the laser based on the horizontal synchronization signal HSYNC and the clock VCLK (see FIG. 10). If the horizontal end position is not detected even when the count value reaches the timing d ′, a horizontal end error signal (horizontal end ERR) is output. At this time, the operation of the rear end detection mode in the CIS 204 is started. The control is the same as when detecting the tip. When the trailing edge of the sheet is detected when the count value reaches timing e, the CIS controller 65 stops the operation of the CIS 204. Even if the count value reaches the timing e ′ (e ′> e), if the trailing edge of the sheet is not detected, the operation of the CIS 204 is forcibly stopped.
[0076]
[Heat shrinkage measurement mode]
FIG. 11 is a flowchart showing the procedure for calculating the contraction rate of the paper in the thermal contraction rate measurement mode. When the thermal contraction rate measurement mode is started by the operation of the operator, the TCU 105 outputs the timing signal described above, transports the paper 107 from the paper feed unit such as the cassettes 34 and 35, and passes through the paper transport path 205 to register. The sheet 107 is temporarily retained in the clutch 203. Then, the registration clutch 203 is turned on, and the sheet 107 is conveyed to the developing unit side (step S1).
[0077]
The TCU 105 acquires the position of the leading edge of the sheet detected by the CIS 204 (step S2), and stores the value in the memory (correction unit parameter storage unit 71) (step S3). When the leading edge detection is completed, the positions of both ends of the sheet in the main scanning direction are acquired from the lateral edge of the sheet detected by the CIS 204 at a predetermined timing (step S4). The value is stored in the memory (step S5). Further, the position of the trailing edge of the sheet detected by the CIS 204 is acquired in the same manner as in the leading edge detection (step 6) and stored in the memory (step S7).
[0078]
After that, the paper length in the main scanning direction is obtained by converting the distance from the positions at both ends of the paper as a result of the lateral edge detection, and further the paper length in the sub-scanning direction is determined from the distance between the leading edge position and the trailing edge position. The outer size of the paper on the first side is measured (step 8). Then, the paper whose outer size is measured is conveyed to the fixing unit 32, and the paper 107 is heated and pressurized by the fixing unit 32, and is passed through the fixing unit 32 (step 9).
[0079]
It is determined whether or not the sheet conveyed from the fixing unit is the front side (first side) (step 10), and if it is determined that the sheet conveyed from the fixing unit is the front side (first side), the sheet In order to measure the thermal contraction rate, the sheet is conveyed to the double-sided conveyance path (circulation path) 206, and is fed again to the registration clutch 203 (step S12). Thereafter, the processing from step 1 to step 9 is repeated. As in the case of the front (front) surface, the outer size of the back surface is measured, and the paper whose outer size is measured is passed through the fixing unit. If it is determined in step S10 that it is the second side, the ratio of the outer size measured on the front surface and the outer size measured on the back surface is determined by the fixing unit in the main scanning direction and the sub-scanning paper direction. The amount of shrinkage (heat shrinkage rate) is calculated (step S11). The calculated shrinkage (heat shrinkage rate) in the main scanning direction and the sub-scanning paper direction is stored in a memory (correction unit parameter storage unit 71) built in the TCU 105. Thereafter, this process is terminated.
[0080]
In this embodiment, the case where the outer size of the first sheet is measured is shown. However, the outer size of the first sheet (front surface) is not measured, and the standard size of the sheet (for example, 297 × 210 mm in the case of A4 size). ) Value may be substituted.
[0081]
[Normal mode]
FIG. 12 is a flowchart showing an image forming process procedure in the normal mode. When the image forming operation in the normal mode is started by the operation of the operator, the TCU 105 outputs the timing signal described above, transports the paper 107 from the paper feed unit such as the cassettes 34 and 35, and passes through the paper transport path 205 to register the clutch. The sheet 107 is once retained in 203. Then, the registration clutch 203 is turned on, and the sheet 107 is conveyed to the developing unit side (step S21).
[0082]
When the leading edge position and the lateral edge position detected by the CIS 204 are acquired (step S22), the TCU 105 performs a laser in the paper feeding (sub-scanning) direction based on the distance L2 between the CIS 204 and the image forming point a. The writing start timing is notified to the laser control circuit 27 (step S23). Further, based on a distance (x + L3) obtained by adding a distance x from the lower end of the CIS 204 to the lateral end position of the sheet to the distance L3 between the CIS 204 and the BD detector 108, the laser writing timing in the main scanning direction is given to the laser control circuit 27. Notification is made (step S24).
[0083]
It is determined whether or not the image formation is on the back side (step S25). If it is determined that the image formation is on the back side, the thermal contraction rate calculated in the above-described thermal contraction rate measurement mode is stored in the memory ( Reading from the correction parameter storage unit 71) (step 26). Write control is performed based on the read value of the thermal contraction rate, and a setting is made to reduce the image in the sub-scanning direction (step 27). Similarly, writing control is performed based on the read thermal contraction rate, and a setting is made to reduce the image in the main scanning direction (step 28). Details of the image reduction processing in the sub-scanning direction and the main scanning direction will be described later.
[0084]
On the other hand, if it is determined in step S25 that the image is formed on the front (front) surface, the image reduction process is not set for heat shrinkage correction in steps S27 and S28, and the process proceeds to step S29 as it is. .
[0085]
At the same time that image reduction processing is set in steps S27 and S28, based on the laser write timing signals in the main scanning direction and sub-scanning direction output from the TCU 105, the laser control circuit 27 outputs a drive signal based on the job to the laser. The image is output to the element (gun) 202 and an image is formed on the sheet 107 (step S29). Then, the sheet 107 is passed through the fixing unit 32, and the image formed on the sheet 107 is fixed by the fixing unit 32 (step S30).
[0086]
It is determined whether or not image formation in the single-side mode or image formation on the back side in the double-side mode is performed (step S31). When the front side image formation is completed in the single-sided mode or the back side image formation in the double-sided mode is finished, the TCU 105 discharges the paper 107 to the finisher side (step S32), and the process is finished. On the other hand, if it is the front side image formation in the duplex mode in step S31, the paper refeeding process is performed (step S33), and the process returns to step S21. Then, after the processes of steps S21 to S31 are performed again, the sheet 107 is discharged to the finisher side (step S32), and this process ends.
[0087]
In this embodiment, a latent image is formed by modulating a laser beam emitted from a laser element (gun) by a laser beam emission circuit (laser control circuit) with an image signal, and the modulated laser beam is a polygon driven by a polygon motor. This is done by raster scanning on the photosensitive drum with the mirror 355. In this embodiment, the following two methods (first image size correction method and second image size correction method) are used as methods for correcting (changing) the image size in the image forming apparatus having such a configuration. Indicates.
[0088]
[First Image Size Correction Method]
FIG. 13 is a diagram showing a configuration of a motor driving device that drives a polygon motor. FIG. 14 is a diagram showing the configuration of the polygon motor control circuit. FIG. 15 is a timing chart showing signal changes in the main part of the motor control circuit.
[0089]
A beam detection signal 304 detected from the beam (BD) detector 108 is a horizontal synchronization signal obtained by detecting the laser beam raster-scanned by the polygon motor 337 at a predetermined position, and after being shaped by the waveform shaping circuit 305. The frequency is divided by two by the frequency divider 307 and input to the falling edge detection circuit 309 and the rising edge detection circuit 315, respectively.
[0090]
The counter a 313 counts the scanner clock (SCNCLK) 310 input from the image formation control circuit (control circuit 51) using the falling edge signal 312 as a starting point. Here, the discrete value 311 is a value obtained by time-converting a predetermined rotation speed input from the image formation control circuit. The counter a 313 starts counting by the input of the falling edge signal 312, and continues counting until the discrete value 311 is reached. For example, the output signal 314 of the counter a 313 is set to be a pulse signal that rises in synchronization with the falling edge 312 signal and falls when it coincides with the discrepancy value 311.
[0091]
Similarly, the counter b 317 counts the scanner clock (SCNCLK) 310 starting from the rising edge signal 316. The counter b 317 starts counting by the input of the rising edge signal 316, and continues counting until the discrete value 311 is reached. The output signal 317 of the counter b 317 is set so as to become a pulse signal that rises in synchronization with the rising edge signal 316 and falls when it coincides with the discrepancy value 311. The OR gate 319 and the NAND gate 321 generate an acceleration signal 320 and a deceleration signal 322 from the output signal 314 of the counter a 313 and the output signal 318 of the counter b 317. Both the acceleration signal (/ ACC) 320 and the deceleration signal (/ DEC) 322 are negative logic signals.
[0092]
For example, when the rotation speed is low, the cycle of the beam detection signal 304 is longer than the discretion value 311, and the difference between the output signal 314 of the counter a and the output signal 317 of the counter b becomes the acceleration signal 320. As the rotational speed of the polygon motor 337 increases, the cycle of the beam detection signal 304 is shortened, and the difference from the discrete value 311 is reduced. Then, the output signal cycle of the acceleration signal 320 is shortened according to the rotational speed of the polygon motor 337. When the rotational speed of the polygon motor 337 exceeds a predetermined rotational speed, the output signal 314 of the counter a and the output signal 317 of the counter b overlap. This overlapping portion is an excess of the rotational speed of the polygon motor 337 and corresponds to the deceleration signal 322.
[0093]
By performing such control, the speed of the polygon motor is stabilized with high accuracy to the target speed (corresponding to the discrepancy value 311). FIG. 16 is a diagram showing the configuration of the laser control circuit 27. The laser control circuit 27 is provided with a laser driver 354, a line buffer 352 including a FIFO, a modulation circuit 351, and the like. The image data synchronized with the data clock (DCLK) 350 is temporarily stored in a line buffer 352 such as a FIFO, then synchronized again with the laser clock (LASERCLK) 353 and sent to the laser driver 354. In some cases, the line buffer is provided with a plurality of lines.
[0094]
The laser driver 354 controls light emission / extinction of the laser element 356 based on image data input in synchronization with a laser clock (LASERCLK) 353. The emitted laser beam is reflected by the surface of the polygon mirror 355 rotated and driven by the polygon motor 337, and scanning of the image for one line is repeated. As a result, a latent image is formed on the photosensitive drum. In the present embodiment, a FIFO is used as the speed conversion line buffer 352, but a LIFO or the like can be selected in accordance with the specifications on the laser side.
[0095]
FIG. 17 is a flowchart showing the reduction setting processing procedure in steps S27 and S28. First, the main scanning reduction ratio Sh and the sub-scanning reduction ratio Sv read in step S26 are acquired (step S41). Here, the reduction ratios Sh and Sv are values of 1 or less. Based on the acquired sub-scanning reduction rate Sv, a scaling in the sub-scanning direction is set (step S42), and further, scaling in the main scanning direction is set based on the acquired main-scanning reduction rate Sh (step S43). As a result of performing the scaling setting in steps S42 and S43, the image before correction is scaled to Sv times in the sub-scanning direction and scaled to Sh (= 1 / Sv × Sv × Sh) times in the main scanning direction. Is obtained (step S44). Then, this process ends. Note that each magnification setting process in steps S42 and S43 will be described later.
[0096]
Here, the reason why the magnification setting in the sub-scanning direction is performed prior to the magnification setting in the main scanning direction is that the magnification setting in the sub-scanning direction is set in the main scanning direction in the sub-scanning direction magnification setting method of the present embodiment. This is because the image in the main scanning direction is also scaled at the same time, affecting the zooming setting. In other words, the main scanning direction and the sub-scanning direction can be independently set by changing the magnification setting in the main scanning direction that does not affect the other magnification setting.
[0097]
FIG. 18 is a flowchart showing the scaling setting processing procedure in the sub-scanning direction in step S42. First, the discrepancy value 311 is set to a value Vini × Sv (where Sv is a value of 1 or less) obtained by multiplying the default motor speed Vini, which is a default value (subscanning direction 100%), by the subscanning reduction ratio Sv (where Sv is a value of 1 or less). Step S51). With this setting, the speed of the polygon motor 337 is accelerated by 1 / Sv times (step S52), and the cycle of the beam detection signal 304 is shortened by Sv times (step S53). As a result, the image in the sub-scanning direction is reduced to Sv times, while the scanning speed in the main scanning is increased to 1 / Sv times, so that the image in the main scanning direction is enlarged to 1 / Sv times. (Step S54). Thereafter, the process returns to step S43.
[0098]
FIG. 19 is a flowchart showing a scaling setting process procedure in the main scanning direction in step S43. In order to reduce the image width in the main scanning direction, the frequency of the laser clock (LASERCLK) 353 is set by the modulation adjustment circuit 351 in consideration of the acceleration of the polygon motor by the scaling setting in the sub-scanning direction as described above. Make it high.
[0099]
That is, the initial frequency fini, which is the value of the default clock (DEFCLK) (100% in the main scanning direction), is divided (= Sv times) by the main scanning enlargement ratio 1 / Sv by the acceleration of the polygon motor, and further the main scanning reduction ratio Sh. The frequency of the laser clock (LASERCLK) 353 is modulated to a value (fini × Sv × Sh) multiplied by (step S55). As a result, the laser scanning speed per pixel is multiplied by (1 / Sv × 1 / Sh) (step S56). As a result, since the scanning cycle in the sub-scanning direction does not change, the image in the sub-scanning direction is not scaled and is set to be reduced to an image of (Sv × Sh) times in the main scanning direction (step S57). Thereafter, the process returns to step S44.
[0100]
The zoom setting is shown with a specific example. When performing duplex copying with a default paper size in the main scanning direction A (7000 pixels) × sub-scanning direction B (3500 pixels), on the first side, the laser clock (LASERCLK) 353 is set to the default frequency fini, and the discretion value 311 Is set to the default motor speed Vini.
[0101]
When the paper size after image fixing on the first side detected by the CIS 204 is C × D, the paper shrinkage ratio is as follows. FIG. 20 is a diagram illustrating image shrinkage correction in the sub-scanning direction. FIG. 21 is a diagram showing the extension of the image in the main scanning direction. FIG. 22 is a diagram showing image shrinkage correction in the main scanning direction. In the figure, Data, Bdata, Cdata, and Ddata represent effective image sizes corresponding to the paper sizes A, B, C, and D, respectively. When shrinking 95% in the main scanning direction, the paper size C = A × 0.95, and when shrinking 97% in the sub-scanning direction, the paper size D = B × 0.97.
[0102]
FIG. 20A shows the effective image size before correction in the sub-scanning direction, and FIG. 20B shows the effective image size after correction in the sub-scanning direction. Since the sub-scanning reduction rate Sv = 0.97, the effective image size in the sub-scanning direction is reduced to 97% by setting the discrepancy value to Vini × 0.97. On the other hand, in the main scanning direction, as shown in FIG. 21, the reflection angle θ per data (pixel) becomes wider by the acceleration ratio. That is, since the main scanning pixel width WidthH is increased by the acceleration ratio, the image is enlarged (100/97)% in the main scanning direction. It is assumed that the sub-scanning reduction rate Sv is a value within a range in which the end data is not missing from the effective image data in the main scanning direction.
[0103]
When performing reduction correction of an image in the main scanning direction, an effective image size before correction in the main scanning direction, that is, after correction in the sub-scanning direction is shown in FIG. The effective image size after correction in the main scanning direction is shown in FIGS.
[0104]
Since the main scanning reduction ratio Sv = 0.95, by modulating the frequency of the laser clock (LASERCLK) 353 to fini × 0.97 × 0.95, only the effective image size in the main scanning direction (97% × 95). %). FIG. 5B means that the enlargement in the main scanning direction by the sub-scan correction is canceled (reduced) by multiplying by the sub-scan magnification of 0.97. Thus, the effective image size in the main scanning direction is reduced to a value obtained by multiplying 95% for the main scanning direction by 97% for the sub scanning direction.
[0105]
[Second Image Size Correction Method]
In the second image size correction method, in addition to the first image size correction method, image data of a predetermined number of pixels (predetermined number of lines) is reduced by thinning out in the main scanning direction (sub-scanning direction). Indicates. FIG. 23 is a flowchart showing a scaling setting process procedure in the sub-scanning direction.
[0106]
First, the number Nv of lines of the selected paper size is acquired (step S71). The sub-scanning reduction rate Sv is acquired from the paper size detected by the CIS 204 (step S72). The number Pv of lines to be thinned out is calculated by the equation (1) (step S73).
[0107]
Pv = Nv × (1-Sv) (1)
Then, it is determined whether or not the number Pv of lines to be thinned is 0 (step S74). If the value is not 0, the line to be thinned out is selected from the array Pv [Nv] (step S75), the number of lines Pv is decremented by 1 (step S76), and the process returns to step S74. Steps S74, S75, and S76 are repeated until the number of lines Pv becomes zero. Thereby, lines to be thinned out are stored in the array Pv [Nv]. When the number of lines Pv becomes 0 in step S74, the lines indicated by the array Pv [Nv] are thinned out (step S77), and this process is terminated.
[0108]
FIG. 24 is a flowchart showing a scaling setting process procedure in the main scanning direction. The number of pixels Nh in the main scanning direction of the selected paper size is acquired (step S61). The sub-scanning reduction ratio Sh is acquired from the paper size detected by the CIS 204 (step S62). The number of pixels Ph to be thinned out is calculated using Equation (2) (step S63).
[0109]
Ph = Nh × (1-Sh) (2)
Then, it is determined whether or not the number of pixels Ph to be thinned is 0 (step S64). If the value is not 0, the number of pixels to be thinned out is selected from the array Ph [Nh] (step S65), the number of pixels Ph is decremented by 1 (step S66), and the process returns to step S64. The processes in steps S64, S65, and S66 are repeated until the pixel number Ph becomes 0. Thereby, the number of pixels to be thinned out is stored in the array Ph [Nh]. When the number of pixels Ph to be thinned out in step S64 becomes 0, the pixels indicated by the array Ph [Nh] are thinned out (step S67), and this process is terminated.
[0110]
In this way, when all the positions of the thinned pixels (lines) are determined, the image is reduced by thinning out the pixels designated by the array Ph [Nh] (lines designated by the array Pv [Nv]). Note that when the reduction ratio is 1, pixel (line) thinning is not required and Ph (Pv) = 0, so the pixels (Pv [Nv]) specified by the array Ph [Nh] (Pv [Nv]) Line) does not exist, and no thinning process is performed.
[0111]
FIG. 25 shows the thinning process. FIG. 6A shows that the paper size before fixing on the first side is A × B. The image data is composed of 7000 pixels × 3500 pixels. FIG. 5B shows that the paper size after fixing on the first side shrunk by heating is C × D.
[0112]
In the figure, arrows indicate the positions of pixels (lines) to be thinned out. Further, numerals (998, 999, 1000, 1001, 1002) attached in the vicinity of the thinned pixels represent pixel numbers. Here, the 1000th pixel is thinned out. When the shrinkage rate (heat shrinkage rate) is as described above, the image size corresponding to the paper size C × D can be corrected by thinning out the number of pixels shown in Equation (3).
[0113]
Number of pixels thinned out in the main scanning direction (number of sub-scanning lines): 350 (= 7000 pixels × {1−0.95})
Number of thinned pixels (number of main scanning lines) in the sub-scanning direction: 305 (= 3500 pixels × {1−0.97}) (3)
In addition, although the position of the pixel to be thinned out is arbitrary, and here, the case where it is equally spaced is shown, it is needless to say that it is not limited and may be changed depending on the image.
[0114]
Further, the method for correcting the image size in the main scanning direction / sub-scanning direction is not limited to the above combination. For example, the image size correction method in the main scanning direction may be laser clock frequency modulation, and the image size correction method in the sub-scanning direction may be image thinning. As described above, the combination of the image size correction methods may be changed according to the main scanning direction / sub-scanning direction, or the image size correction method may be selected according to the magnification.
[0115]
[Second Embodiment]
In the POD (print on demand) market, a test print called a proof print is generally executed before an image of job data sent from a scanner or a PC is formed. Here, the job includes image data input from various input devices and external devices and various settings.
[0116]
Various confirmations such as missing images are performed by this proof print. In the second embodiment, the heat shrinkage is measured at the time of executing the proof print, and the heat shrinkage measurement result at the time of executing the input print job. The case where an image is reduced based on this is shown. The configuration of the image forming apparatus is the same as that in the first embodiment.
[0117]
Here, it is also possible to measure and store the thermal contraction rate of all pages in advance, and to perform image reduction on the back side based on the stored data of all thermal contraction rates. However, since the same material, outer size, and paper in the same direction (orientation) are often loaded in the cassette, it is sufficient to store heat shrinkage rate data for each cassette. To be judged. In 2nd Embodiment, the case where the thermal contraction rate for every cassette is memorize | stored is shown. FIG. 26 is a flowchart showing a proof print processing procedure in the second embodiment.
[0118]
When the proof print process is started, first, one of the cassettes specified in the job is acquired as a cassette n (step S80). Here, the variable n represents a cassette number. It is determined whether or not the paper fed from the designated cassette n is double-sided printing (step S81). In the case of duplex printing, the mode shifts to the thermal shrinkage measurement mode for cassette n (step S82). When determining the dimensional change amount of the sheet, the dimension of the sheet on the front (front) surface serving as a reference is measured (step S83). Here, the size of the paper is detected in each of the main scanning direction and the sub-scanning direction using the line sensor (CIS 204) in the same procedure as in the first embodiment.
[0119]
After the size of the paper is detected, image formation is performed on the front surface of the paper (step S84). The fixing operation in this process causes thermal contraction of the paper. The heat-shrinked paper is fed again to the image forming unit via the duplex conveyance path (step S85). At this time, before forming an image on the back surface, the size of the paper is detected (step S86), and compared with the previously obtained surface (front) surface size, the change rate is calculated and stored (step). S87). Thereafter, image formation is performed on the back surface (step S88). At this time, in the proof print, it is not necessary to perform image magnification correction (size correction).
[0120]
Thereafter, it is determined whether or not there is another designated cassette in the job (step S89). If there is another designated cassette, the process returns to step S80, the number of the other designated cassette is substituted into the variable n, and the same processing is repeated. On the other hand, if there is no other designated cassette, the process is terminated.
[0121]
On the other hand, if the paper fed from the cassette designated in step S81 is not double-sided printing, front side image formation is performed (step S90), and the process proceeds to step S89.
[0122]
As described above, when a plurality of cassettes are designated for a job, the thermal contraction rate is obtained for each cassette, the thermal contraction rate is stored as data corresponding to each cassette, and these data are used during job printing. . Thus, the thermal contraction rate is calculated by repeating the number of cassettes designated by the proof print.
[0123]
FIG. 27 is a flowchart showing a normal job print processing procedure. When the image forming operation in the normal mode is started by the operation of the operator, the TCU 105 outputs the timing signal described above. When the paper cassette to be fed is the cassette n, the paper 107 is transported from the paper feeding unit of the cassette n. (Step S101). Then, the sheet 107 is temporarily retained in the registration clutch 203 through the sheet conveyance path 205, the registration clutch 203 is turned on, and the sheet 107 is conveyed to the developing unit side (step S102).
[0124]
In the image formation on the front surface, heat shrinkage due to the fixing operation has not yet occurred, so that normal image formation is performed without correcting the magnification of the image (step S103). Thereafter, heat shrinkage occurs during the fixing operation (step S104). It is determined whether or not double-sided printing is performed on the fed paper (step S105).
[0125]
Further, if it is determined that double-sided printing is to be performed on the fed paper, refeeding is performed (step S106), and the cassette n stored at the time of proof printing is performed before image formation after refeeding is performed. The heat shrinkage rate is read (step S107). Then, through the paper transport path 205, the re-feed paper 107 is temporarily retained in the registration clutch 203, the registration clutch 203 is turned on, and the paper 107 is transported to the developing unit side (step S108). Based on the data of the thermal contraction rate, the scaling setting is performed in the main scanning direction and the sub-scanning direction, and an image is formed on the back surface of the sheet (step S109). On the other hand, if double-sided printing is not performed in step S105, the process proceeds directly to step S110.
[0126]
It is determined whether or not to end the printing operation (step S110). When the printing operation is to be ended, this processing is ended. On the other hand, if the printing operation is not terminated, the process returns to step S101. If the paper to be fed again is another cassette, the feeding operation is similarly performed from cassette n with the number of the cassette as the variable n. Even when the paper is fed from another cassette n, similarly, based on the heat shrinkage rate of the cassette n previously obtained, the magnification setting in the main scanning direction and the sub-scanning direction is performed to correct the image size. These printing operations are repeated until image formation on all sheets is completed.
[0127]
As described above, according to the present embodiment, when forming images on both sides of the paper, the image sizes in the main scanning direction and the sub-scanning direction are accurately corrected in accordance with the change rate of the paper in the main scanning direction and the sub-scanning direction. it can. That is, by using the line sensor (CIS 204) to detect the thermal shrinkage rate in the sub-scanning direction, it is possible to detect a highly accurate thermal shrinkage rate (change rate) that cannot be obtained conventionally, and also in the main scanning direction. The thermal shrinkage rate (change rate) can be detected by the same line sensor, and the image size is accurately corrected based on the heat shrinkage rates in the main scanning direction and the sub-scanning direction obtained with high accuracy. Can do.
[0128]
In addition, the image size can be appropriately reduced with respect to various heat shrinkage rates that vary greatly depending on humidity, material, and direction of tightening. As a result, it is possible to solve the problem that the positional deviation and image size of the image are different between the front surface and the back surface, and it is possible to obtain a product with high commercial value and good appearance.
[0129]
Although the case where the thermal contraction rate is obtained for each cassette is shown here, when the material is specified by the setting from the operation unit or the like, the thermal contraction rate is obtained and stored at the time of proof printing for each set material. Alternatively, correction may be performed based on the value.
[0130]
In the present embodiment, a contact image sensor is shown as a typical line sensor. Since the accuracy of this CIS is typically 600 dpi, it can theoretically be detected with a resolution of 0.042 mm. Therefore, the shrinkage rate of the paper can be detected with high accuracy. In addition, a single line sensor is sufficient for the main scanning direction and the sub-scanning direction, which is very advantageous in terms of cost and mounting.
[0131]
The above is the description of the embodiments of the present invention, but the present invention is not limited to the configurations of these embodiments, and the functions shown in the claims or the functions of the configurations of the embodiments are achieved. Any configuration that can be applied is applicable.
[0132]
For example, in the above embodiment, after detecting the timings in the main scanning direction and the sub-scanning direction, the TCU 105 is notified of these timings. However, the adjustment of the laser writing timing after the detection is not particularly limited. Any adjustment method may be used. Further, although the image forming timing in the sub-scanning direction is determined by detecting the leading edge of the paper, it may be determined by detecting the trailing edge of the paper by CIS depending on the mechanical configuration of the apparatus. Further, the measurement process in the heat shrink mode may be performed at any timing.
[0133]
In the above embodiment, the fixing method is the heat fixing method, and the case where the thermal contraction rate of the paper that thermally contracts after fixing is detected. However, the fixing method is not limited to this, and the pressure fixing method is used. In the case of thermal expansion after fixing, the thermal expansion coefficient of the paper is detected. Then, the size of the image formed on the paper is corrected according to the coefficient of thermal expansion.
[0134]
In the above embodiment, the sheet change rate is measured for each paper feed cassette. However, when the paper used for each job is assumed to be different, the paper change rate is measured for each job. Also good. Further, the rate of change may be measured for each type of paper (for example, material, size, paper orientation, coloring presence / absence, texture direction, etc.).
[0135]
Also, the rate of change of the paper is measured for each page, the measured rate of change of the paper is stored for each page, and when image formation is performed for each page, the image size is based on the stored rate of change of the page. May be changed.
[0136]
Embodiments of the present invention are listed below.
[0137]
[Embodiment 1] Image forming means for forming an image on a sheet, and conveying the sheet to the image forming means on the front surface, and forming the image again with the sheet on which the image is formed on the front surface by the image forming means. An image forming apparatus that forms images on both sides of the sheet, and a plurality of reading pixels arranged in a passage region so as to be aligned in the width direction of the sheet; and Main scanning direction change rate detecting means for detecting a change rate of a dimension in the main scanning direction of the sheet conveyed on the back surface by reading the reading pixels, and a sub-portion of the sheet conveyed on the back surface by reading the reading pixels. Sub-scanning direction change rate detecting means for detecting a change rate of the dimension in the scanning direction and the image formation based on the detected change rate of the dimension in the main scanning direction of the sheet A main scanning direction magnification correcting means for correcting the magnification in the main scanning direction of the image formed on the back surface of the sheet by the means, and the image forming means based on the change rate of the detected dimension in the sub scanning direction. An image forming apparatus comprising: a sub-scanning direction magnification correcting unit that corrects a magnification in the sub-scanning direction of an image formed on the back surface of the image forming apparatus.
[0138]
[Embodiment 2] The sheet reading unit includes a line sensor in which a plurality of reading pixels are arranged in a conveyance path of the sheet at a size equal to or larger than a dimension in the main scanning direction of the conveyed sheet, and the main scanning direction change rate detection is performed. The means detects a change rate of the dimension in the main scanning direction of the sheet from a dimension in the main scanning direction of the sheet read when passing through the line sensor, and the sub-scanning direction change rate detection means detects the line sensor. 2. The image forming apparatus according to claim 1, wherein a rate of change in dimension of the sheet in the sub-scanning direction is detected from the positions of the leading edge and the trailing edge of the sheet that are read when the sheet passes.
[0139]
[Embodiment 3] The image forming apparatus includes a fixing unit that fixes an image formed on the sheet, and the main scanning direction change rate detection unit and the sub-scanning direction change rate detection unit each have dimensions in the main scanning direction of the sheet before and after fixing. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is detected from a change in the size and a size change in the sub-scanning direction.
[0140]
[Embodiment 4] The conveying means feeds the sheet storing means for stacking the sheets, and the sheets stacked on the sheet storing means from the sheet storing means to the image forming means through a feeding path. Sheet feeding means, and sheet refeeding means for causing the sheet on which an image is formed by the image forming means to pass through a circulation path to be a back surface, and joining the sheet that has become the back surface to the feeding path again. The image forming apparatus according to claim 2, wherein the line sensor is disposed in the feeding path to the image forming unit.
[0141]
[Embodiment 5] The main scanning direction magnification correction unit corrects the magnification in the main scanning direction of an image formed on the back surface of the sheet by the image forming unit by thinning out pixels in the main scanning line at a predetermined interval. An image forming apparatus according to Embodiment 1, wherein
[0142]
[Embodiment 6] The sub-scanning direction magnification correction unit corrects the magnification in the sub-scanning direction of an image formed on the back surface of the sheet by the image forming unit by thinning out main scanning lines at a predetermined interval. The image forming apparatus according to Embodiment 1, wherein the image forming apparatus is characterized in that
[0143]
[Embodiment 7] When the image forming unit irradiates the photosensitive drum with laser light in synchronization with a reference clock to form a latent image, the main scanning direction magnification correction unit changes the frequency of the reference clock. The image forming apparatus according to Embodiment 1, wherein the magnification in the main scanning direction of the image formed on the back surface of the sheet is corrected.
[0144]
[Embodiment 8] When the image forming unit irradiates the photosensitive drum while scanning the photosensitive drum with laser light reflected by the polyhedral mirror to form a latent image, the magnification in the sub-scanning direction is used. 2. The image forming apparatus according to claim 1, wherein the correcting unit corrects the magnification in the sub-scanning direction of the image formed on the back surface of the sheet by controlling the rotational speed of the polyhedral mirror.
[0145]
[Embodiment 9] The image forming means emits laser light toward a polyhedral mirror in synchronization with a reference clock, and rotates the polyhedral mirror to cause the laser light reflected by the polyhedral mirror to be incident on the photosensitive drum. When irradiating while scanning to form a latent image, the sub-scanning direction magnification correction unit controls the rotation speed of the polyhedral mirror, thereby sub-scanning the image formed on the back surface of the sheet by the image forming unit. After correcting the magnification in the direction, the main scanning direction magnification correction means corrects the magnification in the main scanning direction of the image formed on the back surface of the sheet by changing the frequency of the reference clock. The image forming apparatus according to the first embodiment.
[0146]
[Embodiment 10] The image forming apparatus according to Embodiment 3, wherein the dimensions of the sheet before fixing in the main scanning direction and the sub-scanning direction are specified values.
[0147]
[Embodiment 11] Embodiment 3 wherein the main scanning direction change rate detecting means and the sub scanning direction change rate detecting means measure dimensions in a main scanning direction and a sub scanning direction of a sheet before fixing. Image forming apparatus.
[0148]
[Embodiment 12] When the fixing unit performs fixing by heating, the main scanning direction change rate detection unit and the sub-scanning direction change rate detection unit detect the shrinkage rate of the sheet. The image forming apparatus according to Aspect 3.
[0149]
[Embodiment 13] When the fixing unit performs fixing by applying pressure, the main scanning direction change rate detecting unit and the sub-scanning direction change rate detecting unit detect an expansion rate of the sheet. The image forming apparatus according to Embodiment 3.
[0150]
[Embodiment 14] The image forming means adjusts the image formation start position in the sub-scanning direction based on the position of the leading edge or the trailing edge of the sheet read by the line sensor. The image forming apparatus described.
[0151]
[Embodiment 15] The embodiment is characterized in that the image forming means adjusts the image forming start position in the main scanning direction based on the position of the lateral edge of the sheet read by the line sensor. Image forming apparatus.
[0152]
[Embodiment 16] When an image is formed in accordance with execution of an input job, the main scanning direction change rate detecting means and the sub-scanning direction change rate detecting means until the image is formed after the job is input. 2. The image forming apparatus according to claim 1, wherein a change rate of a dimension in the main scanning direction and a change rate of a dimension in the sub-scanning direction of each of the sheets are detected during each of the periods.
[0153]
[Embodiment 17] When a plurality of jobs are input, the main scanning direction change rate detection means and the sub-scanning direction change rate detection means respectively change the size change rate and the sub-size of the sheet in the main scanning direction for each job. 17. The image forming apparatus according to embodiment 16, wherein a rate of change in dimension in the scanning direction is detected.
[0154]
[Embodiment 18] A plurality of sheet storing means for stacking sheets are provided, and the main scanning direction change rate detecting means and the sub-scanning direction change rate detecting means are respectively provided for each sheet storing means used for executing the job. 17. The image forming apparatus according to claim 16, wherein a rate of change in dimension in the main scanning direction and a rate of change in dimension in the sub-scanning direction of the sheet are detected.
[0155]
[Embodiment 19] The main-scanning direction magnification correction unit includes storage means for storing a change rate of the dimension in the main scanning direction and a change rate of the dimension in the sub-scanning direction detected for each sheet storage unit. And the sub-scanning direction magnification correcting unit is formed on the back surface of the sheet by the image forming unit based on the stored change rate of the dimension in the main scanning direction and the change rate of the dimension in the sub-scanning direction, respectively. 19. The image forming apparatus according to embodiment 18, wherein the magnification of the image in the main scanning direction and the sub scanning direction is corrected.
[0156]
[Embodiment 20] The main scanning direction change rate detection means and the sub-scanning direction change rate detection means respectively change the dimensional change rate of the sheet in the main scanning direction for each type of sheet used to execute the job. The image forming apparatus according to Embodiment 16, wherein a rate of change in dimension in the sub-scanning direction is detected.
[0157]
[Embodiment 21] The image forming apparatus according to Embodiment 20, wherein the sheet type includes at least one of material, size, and sheet orientation.
[0158]
[Embodiment 22] Storage means for storing the change rate of the dimension in the main scanning direction and the change rate of the dimension in the sub scanning direction of the sheet for each page is provided, and the main scanning direction change rate detecting means and the sub scanning direction change are provided. The rate detection means detects the change rate of the dimension in the main scanning direction and the change rate of the dimension in the sub-scanning direction for each page, stores the detection result in the storage means, and stores the detection result in the main scanning direction. The magnification correction unit and the sub-scanning direction magnification correction unit are respectively controlled by the image forming unit based on the stored change rate of the dimension in the main scanning direction and change rate of the dimension in the sub-scanning direction for each page. 18. The image forming apparatus according to claim 17, wherein the magnification formed in the main scanning direction and the sub scanning direction of the image formed on the back surface of the sheet is corrected.
[0159]
[Embodiment 23] A sheet is conveyed on the front surface to an image forming means for forming an image on the sheet, and after the image is formed on the surface of the sheet by the image forming means, the sheet is conveyed to the image forming means again with the back surface. In the image forming method in which an image is formed on both sides of the sheet by forming an image on the back side of the sheet by the image forming unit, a change rate of a dimension in the main scanning direction of the sheet conveyed on the back side A main scanning direction change rate detecting step for detecting the sheet, a sub-scanning direction change rate detecting step for detecting a change rate of a dimension of the sheet conveyed on the back surface in the sub-scanning direction, and the detected sheet in the main scanning direction. A main scanning direction magnification correction step for correcting a magnification in the main scanning direction of an image formed on the back surface of the sheet by the image forming unit based on a rate of change in dimensions; And a sub-scanning direction magnification correcting step for correcting a magnification in the sub-scanning direction of an image formed on the back surface of the sheet by the image forming unit based on a change rate of a dimension in the sub-scanning direction of the sheet. Image forming method.
[0160]
【The invention's effect】
According to the present invention, when forming images on both sides of a sheet, the image size can be accurately corrected in accordance with the change rate of the sheet size in the main scanning direction and the sub-scanning direction. Further, by detecting the change rate of the sheet size in both the main and sub-scanning directions with a single (same) line sensor, it is very advantageous in terms of cost and mounting.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of an image forming apparatus according to a first embodiment.
FIG. 2 is a diagram illustrating a print position adjustment mechanism disposed in a paper conveyance path that reaches a photosensitive drum.
FIG. 3 is a diagram showing a configuration of a CIS 204.
FIG. 4 is a diagram illustrating an arrangement of a CIS 204 with respect to a paper passage area.
FIG. 5 is a diagram illustrating a method for detecting a sheet size in a main scanning direction and a sub-scanning direction using a single CIS.
FIG. 6 is a block diagram showing a configuration of a control circuit.
7 is a block diagram showing a configuration of a TCU 105. FIG.
FIG. 8 is a block diagram showing a configuration of a tip detection unit 63;
9 is a timing chart showing the operation of the TCU 105. FIG.
FIG. 10 is a diagram showing write position adjustment by a laser.
FIG. 11 is a flowchart showing a procedure for calculating a shrinkage ratio of a sheet in a heat shrinkage ratio measurement mode.
FIG. 12 is a flowchart illustrating an image forming process procedure in a normal mode.
FIG. 13 is a diagram illustrating a configuration of a motor driving device that drives a polygon motor.
FIG. 14 is a diagram showing a configuration of a polygon motor control circuit.
FIG. 15 is a timing chart showing signal changes in the main part of the motor control circuit;
16 is a diagram showing a configuration of a laser control circuit 27. FIG.
FIG. 17 is a flowchart showing a reduction setting processing procedure in steps S27 and S28.
FIG. 18 is a flowchart showing a scaling setting process procedure in the sub-scanning direction in step S42.
FIG. 19 is a flowchart showing a scaling setting process procedure in the main scanning direction in step S43.
FIG. 20 is a diagram illustrating image shrinkage correction in the sub-scanning direction.
FIG. 21 is a diagram illustrating the expansion of an image in the main scanning direction.
FIG. 22 is a diagram illustrating image shrinkage correction in the main scanning direction.
FIG. 23 is a flowchart showing a scaling setting process procedure in the sub-scanning direction.
FIG. 24 is a flowchart illustrating a scaling setting processing procedure in the main scanning direction.
FIG. 25 is a diagram illustrating a thinning process.
FIG. 26 is a flowchart showing a proof print processing procedure in the second embodiment.
FIG. 27 is a flowchart illustrating a normal job print processing procedure.
FIG. 28 is a diagram illustrating a positional deviation between a conventional image and a sheet.
FIG. 29 is a diagram showing a result of copying the back surface while being displaced by a preset amount.
FIG. 30 is a diagram illustrating a state in which a change in the paper in the sub-scanning direction is detected by a sensor (mechanical flag sensor) using a mechanical detection lever.
[Explanation of symbols]
27 Laser control circuit
31 Photosensitive drum
51 Control circuit
105 TCU
107 paper
202 Laser element
204 CIS
205 Paper transport path
206 Circular path
310 Scanner clock (SCNCLK)
311 Discrete value
337 polygon motor
355 polygon mirror

Claims (4)

An image forming unit that forms an image on a sheet, and a sheet that conveys the sheet to the image forming unit on the front surface, and a sheet that has an image formed on the front surface by the image forming unit and that conveys the sheet to the image forming unit again An image forming apparatus for forming images on both sides of the sheet,
A sheet reading means arranged in a passing region so that a plurality of reading pixels are arranged in the width direction of the sheet;
A main scanning direction change rate detecting means for detecting a change rate of a dimension in the main scanning direction of the sheet conveyed on the back surface by reading the reading pixels;
A sub-scanning direction change rate detecting means for detecting a change rate of a dimension in a sub-scanning direction of the sheet conveyed on the back surface by reading the reading pixels;
Main scanning direction magnification correction means for correcting the magnification in the main scanning direction of the image formed on the back surface of the sheet by the image forming means based on the detected change rate of the dimension in the main scanning direction of the sheet;
A sub-scanning direction magnification correction unit that corrects a magnification in the sub-scanning direction of an image formed on the back surface of the sheet by the image forming unit based on the detected change rate of the dimension in the sub-scanning direction ;
The sheet reading means includes
Among the plurality of reading pixels, a plurality of chips each storing a number of reading pixels divided in the main scanning direction;
Selecting means for selecting at least one of the plurality of chips;
The image forming apparatus according to claim 1, wherein the main scanning direction change rate detection unit and the sub-scanning direction change rate detection unit respectively read out reading pixels in a chip separately selected by the selection unit. When detecting the change rate of the dimension in the sub-scanning direction, the sub-scanning direction change rate detecting unit reads out the reading pixel in any one of the plurality of chips selected by the selecting unit. The image forming apparatus according to claim 1. When detecting the change rate of the dimension in the main scanning direction, the main scan direction change rate detecting means is the remaining chip except for the chip selected when detecting the change rate of the dimension in the sub-scanning direction. 3. The image forming apparatus according to claim 2, wherein the reading pixels in at least two chips selected by the selection unit are read out. The sub-scanning direction change rate detection means reads the reading pixels in one chip selected by the selection means to detect the leading edge of the sheet,
  The main scanning direction change rate detection means reads the reading pixels in at least two chips selected by the selection means after detecting the leading edge of the sheet, and detects both ends of the sheet in the main scanning direction. Detect
  The sub-scanning direction change rate detection means reads out the reading pixels in one chip selected by the selection means after detecting both ends of the sheet in the main scanning direction, and detects the trailing edge of the sheet. Detect
  The sub-scanning direction change rate detecting means obtains the dimension of the sheet in the sub-scanning direction from the detected leading edge and trailing edge of the sheet,
  The image forming apparatus according to claim 3, wherein the main scanning direction change rate detection unit obtains a dimension of the detected sheet in the main scanning direction from both ends of the detected sheet in the main scanning direction.

Images